a. Field of the Invention
The present disclosure relates generally to ablation systems, and more particularly, to a method and apparatus for radiofrequency ablation with increased depth and/or decreased volume of ablated tissue.
b. Background Art
It is known to deliver radiofrequency (RF) energy to a desired target area through an electrode assembly to ablate tissue at the target site. RF ablation may generate significant heat, which if not controlled can result, generally, in undesired or excessive tissue damage. It is particularly difficult to achieve successful RF ablation of live, relatively thick biological tissue, for a number of reasons. The cross-sectional profile (i.e., depth) of ablation is often too shallow, in part because most of the Ohmic heat is generated near the RF electrodes, i.e., at the surface of the tissue. Furthermore, the rate at which heat diffuses deep into the tissue layer(s) is extremely slow, and may be counteracted by the cooling effects due to blood perfusion, for example, during epicardial ablation. As the tissue surrounding the electrodes is being ablated, its Ohmic resistivity increases. This increase in resistivity further exacerbates the problem of non-uniform heating, because at a given current density, the heating intensity experienced by the tissue is proportional to its then-existing Ohmic resistivity.
Moreover, the goals of thick layer RF cardiac ablation are often at odds. It is often necessary to ablate all the way through the thickness of the tissue, while at the same time avoiding tissue overheating and minimizing the volume of ablated tissue. However, the thicker the tissue layer, the less possible it is to achieve adequate ablation through its thickness without overheating the tissue near the RF electrodes (i.e., damaging the tissue in an unacceptable way) and without ablating an undesirable excess volume of tissue.
There are a number of two-electrode or multi-electrode RF ablating devices known in the art, for example as seen by reference to U.S. Pat. No. 7,241,292 entitled CARDIAC ABLATION DEVICE WITH MOVABLE HINGE issued to Hooven. Hooven discloses a transmural RF ablation device. However, Hooven does not address the problems of inadequate ablation depth or excessive ablated tissue volume described above, particularly for a relatively thick biological tissue layer.
There is therefore a need to minimize or eliminate one or more of the problems set forth above.